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New Member

Slow FTP transfer

Hi,

We have a network between two locations. WAN is 100 Mbps MPLS provided by service provider. There is a FWSM module on core switches.

Problem we are facing is FTP file transfer between two servers across WAN link never crosses 20 Mbps. Link is 100 Mbps. However if we do parallel file tranfers each transfer is 20 Mbps . But we dont get transfer rate above 20 Mbps in a single FTP session. Is there any bottle neck where traffic is getting restricted.

Traffic between two mservers passes through 100Mbps MPLS WAn link , 6500 core switch and FWSM module in switch.

Thanks

KP

  • WAN Routing and Switching
7 REPLIES
Super Bronze

Slow FTP transfer

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

TCP will self limit its transmission rate if the receiver's RWIN is not at least sized for the BDP (bandwidth delay product).

If for example, your latency was 100 ms, for 100 Mbps you would need 100,0000,000 * .1 / 8 bytes (1.25 MB).

New Member

Slow FTP transfer

Thanks Joseph !

I like to understand further on this. By receiver do you mean the server or the router/firewall. We are using FTP on AIX servers.

Regards

KP

Super Bronze

Slow FTP transfer

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

I like to understand further on this. By receiver do you mean the server or the router/firewall. We are using FTP on AIX servers.

Actual host system that is receiving the FTP.

If you google BDP, you'll find more info about this, and you can also google for information on setting various host OSs.

BTW, suspect what Milan described was the same issue.  Even with a properly set RWIN, multiple FTP streams will likely transfer the data faster because parallel stream will each ramp-up (TCP slow start).  This is especially noticeable if a particular file is small as even with ideal settings the flow might not ramp up to full speed before the transfer is completed.

Slow FTP transfer

Hi Joseph,

I believe multiple FTP streams might also be faster in a case of line error recovery under some conditions (like SACK not supported by the FTP client).

BR,

Milan

Super Bronze

Slow FTP transfer

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

I believe multiple FTP streams might also be faster in a case of line error recovery under some conditions (like SACK not supported by the FTP client).

SACK should be a function of the TCP stack, not the application, but yes multiple FTP stream would recover faster, with or without SACK, again because each one does recovery in parallel.  Further, with multiple streams not all may be in recovery from packet lost at the same time or at the same flow rate state.

PS:

There are a couple of file transfer packages, I believe, that will automatically spawn off concurrent flows for a file transfer.  There are others that will try to optimize the transfer, in various other ways such as using compression and/or data block deltas.  Some might use multiple techniques.

In other words, although adjusting the receiving host's RWIN might allow a single TCP flow to utilize all the bandwidth, on an LFN, other techniques for file transfer can be much more optimal.

Slow FTP transfer

Hi,

you possibly reached an FTP rate limit caused by the ACK rount trip delay.

I remember similar problem some years ago when we were transferring large files from Germany to India via FTP.

A solution that time was using a multi-thread FTP client.

HTH,

Milan

New Member

Slow FTP transfer

My advice just for testing purposes is to generate UDP traffic among sites and use a QoS scheme. You can see if there is any congestion in the network.

Beside this I must leave here a link for a perfect explanation of BDP:

http://bradhedlund.com/2008/12/19/how-to-calculate-tcp-throughput-for-long-distance-links/

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